1. Field of the Invention
The present invention relates to an image forming device for forming images using electronic printing technology, and more particularly to an image forming device which has a plurality of development units containing a same color developer and can simplify calibration by patch patterns.
2. Description of the Related Art
An image forming device for forming images using electro-photographic technology is installed in a printer, facsimile and copier, and is comprised of an image carrier (photosensitive drum) on which an electrostatic latent image is formed according to the image data, a charge unit for charging the outer face of the image carrier, an exposure unit for exposing the charged outer face of the image carrier according to the image data and forming an electrostatic latent image, a development device for developing the electrostatic latent image of the image carrier into a toner image by supplying toner, which is the developer, to the image carrier, and a transfer unit for transferring the toner image to the transfer target medium. The development device detachably holds a plurality of development units which contain color toners and moves an appropriate development unit closer to the image carrier according to the development timing. For this, the development device has a development rotary of which rotation is controlled. For color printing, the development units of a plurality of color toners, such as four colors (yellow Y, magenta M, cyan C and black K) are installed in the development rotary, and these development units are sequentially moved closer to the image carrier, and each color is developed.
On the other hand, an image forming device, dedicated to monochrome printing by simultaneously installing a plurality of development units with the same color, such as black, in the development rotary of the development device, has been proposed. Examples are Japanese Patent Application Laid-Open No. 2002-351190 (Dec. 4, 2002) and Japanese Patent Application Laid-open No. 2003-316106 (Nov. 6, 2003). Since in this image forming device dedicated to monochrome printing, a plurality of black development units can be installed, so the replacement frequency of development units can be decreased by sequentially using the plurality of development units even if a large volume of monochrome printing is performed.
In the case of an image forming device using electro-photographic technology, the image carrier is charged to a predetermined bias potential, a latent image is formed by irradiating an exposure beam with a predetermined intensity, and toner is attached from the development unit by the bias voltage difference between the development unit and image carrier. Therefore the toner attachment status differs depending on such control parameters as the bias voltage and the exposure intensity between the development unit and image carrier. Even if the same control parameters are used, the toner attachment status differs by changes in the external environment and the replacement of the development unit. Therefore calibration is normally performed where a predetermined patch pattern is generated on the image carrier by the toner, and optimum control parameters are determined according to the optical density of the patch pattern.
Optimum control parameters may differ depending on the positional relationship between the development unit and the image carrier, and the status and type of the developer in the development unit, and in the case of a color image forming device which superimposes images of a plurality of colors of developer, calibration by a patch pattern is performed for all the development units. This is because in the case of color images, if the optical density of each color disperses, the mixed color thereof itself becomes different.